# Patent

## Title
Compensation circuit and compensation method to compensate nonlinear distortions of an A/D converter

## Abstract

The invention relates to a compensation circuit (1–6) to compensate nonlinear distortions of an A/D converter comprising a signal input and a compensation system. In order to avoid the high costs related to the complex analog design of the A/D converter and/or the high energy consumption in this converter and compensation system, it is proposed that a compensation circuit be used that is composed of digital circuit elements that are connected following the A/D converter and which has a nonlinearly distorted output signal of the A/D converter supplied to it to compensate the nonlinear distortion. Adaptive coefficients are preferably employed here for the compensation.

## Background
BACKGROUND OF THE INVENTION
The invention relates to the field of analog-to-digital signal conversion, and in particular to a compensation circuit to compensate nonlinear distortions of an analog-to-digital (A/D) converter.
A/D converters are a critical component in integrated circuits having mixed signal processing (i.e., analog and digital signal processing). Requirements related to the linearity of the A/D converter are quite difficult to achieve given the usual tolerances for analog components. The measures required for this purpose in the area of analog design entail high cost and/or high current consumption by the circuit.
In order to prevent nonlinear distortions in the analog-to-digital conversion process of the A/D converter, compensation circuits are employed to compensate for these nonlinear distortions of the A/D converter. The compensation circuits have an analog signal input, and are typically located on the input side of the A/D converter.
There is a need for a compensation circuit that compensates for nonlinear distortions of an A/D converter, and requires a simplified overall analog design together with, preferably, reduced current consumption.
SUMMARY OF THE INVENTION
A compensation circuit to compensate for nonlinear distortions of an A/D converter may include a signal input and a compensation circuit composed of digital circuit elements to digitally compensate for the nonlinear distortions, the signal input as the compensation input being a digital signal input to supply a signal outputted in distorted form by the A/D converter. Implementation is thus in the form of an output-side digital compensation or distortion correction of the nonlinear distortions from an input-side A/D converter. The digital circuit elements required for this purpose are inexpensive and readily available based on a simple digital circuit design. Advantageously, it is no longer necessary to incur high costs or have increased current consumption based on a compensation circuit added on the input-side of, or integrated into, the A/D converter.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustration of a compensation circuit to compensate for nonlinear distortions of an A/D converter;
FIG. 2 is a block diagram illustration of a circuit to determine a test signal;
FIG. 3 is a block diagram illustration of a circuit for the iterative calculation of correction coefficients; and
FIG. 4 is a block diagram illustration of an alternative circuit for the iterative calculation of correction coefficients using a look-up table.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram illustration of a compensation circuit to compensate for nonlinear distortions of an A/D converter. The compensation circuit in the example is composed of an analog section, shown at left, and a digital section shown on the right. An analog input signal x(t) is supplied to an A/D converter (ADC) 1, which provides a digital sequence xn corresponding to the analog input signal x(t). When a conventional ADC 1 is used, the digital sequence xn from this converter exhibits nonlinear distortion. The index n represents the sequence of sampling values xn, n=0, 1, 2, . . . .
The A/D-converted sequence xn is input of a compensation circuit 2, which compensates or corrects the nonlinear distortion created by the ADC 1. The compensation circuit 2 provides compensated digital data yn.
The compensation circuit 2 receives coefficients c1, c2, . . . cK, c0 that have been determined or calculated based on the nonlinear distortion response of the ADC 1. The index k=1, 2, . . . , K here functions as the consecutive index for the coefficients ck of compensation.
The circuit of this design includes an ADC 1 together with the compensation circuit 2 to which a set of coefficients c1, . . . cK is supplied offers a simple design which enables an analog-to-digital conversion of an analog signal x(t) to form a sequence of compensated digital data yn which does not suffer from nonlinear distortion.
A coefficient computation unit 100 determines the coefficients c1, . . . cK. These additional components are advantageously active only during a configuration phase. As an alternative to the circuit described below, it is also possible to employ a memory in which a set of coefficients c1, . . . cK, determined previously only once, is stored which may be applied generally for the compensation. However, the preferred approach is the circuit design described below which provides an adjustment of the coefficients c1, . . . cK to the actual and/or instantaneous conditions.
Referring to FIG. 1, a test signal s(t) is generated by a test signal generator 3 and applied during the configuration phase to the input of the ADC converter 1. The test signal generator 3 also provides either the parameters sn to generate the analog test signal s(t) or a sequence of digital test signal data Sn corresponding to this analog test signal s(t). These parameters sn or test signal data Sn are fed to a coefficient determination system 5 and/or to a test signal check device 4.
The coefficient determination system 5 determines the coefficients c1, . . . cK to be used by the compensation circuit 2. The coefficient determination system 5 receives the digital data xn from the ADC 1. The coefficient determination system 5 also receives distortion data or difference data Dn on a line 102. The difference data Dn are provided by a subtracter 6 to which the sequence of the compensated digital data yn is fed from the output of the compensation circuit 2. The subtracter 6 also receives a sequence of digital signal data Sn either directly or indirectly through the test signal check device 4. The sequence of digital signal data Sn corresponds in the configuration phase to a signal map of the test signal, whereby, after appropriate determination of coefficients has been effected, this signal map is as undistorted as possible, or ideally is completely undistorted.
During the configuration phase, the analog test signal s(t) is generated and fed to the ADC 1, which provides the sequence of digital data xn to the compensation circuit 2 and the coefficient determination circuit 5. If the set of coefficients c1, . . . cK is not available, the compensation circuit 2 outputs the sequence of digital data xn as the sequence of compensated digital data yn(i.e., yn=xn). This data is fed to the subtracter 6, which is also supplied with a corresponding sequence of digital test signal data Sn matching an undistorted data set. After subtraction of the two sequences of data (yn−Sn), the data sequence of the difference signal Dn on a line 102 is supplied to the coefficient determination system 5. According to a preferred embodiment, a test signal s(t) is subsequently sent by the test signal generator 3, which may also be composed of a memory with an analog test signal s(t) and a corresponding digital parameter set sn, to the ADC 1. The sequence of digital data xn generated by the ADC 1 is subsequently compensated by the compensation circuit 2 in accordance with this supplied set of coefficients c1, . . . cK, such that the sequence of compensated digital data yn is output, ideally with already optimized coefficients c1, . . . cK, without nonlinear distortion. The sequence of compensated digital data yn is in turn fed to the subtracter 6 in which, after subtraction using the corresponding values of the sequence of digital test signal data Sn, the sequence of data Dn of the difference signal is again generated. This difference data Dn is again fed to the coefficient determination system 5 which, in the event the difference data Dn does not equal zero or exceeds predetermined threshold values, implements another, or preferably, an iteratively improved determination of coefficients to provide improved coefficients c1, . . . cK.
After a sufficiently distortion-corrected or compensated set of coefficients c1, . . . cK has been determined, the configuration phase ends, after which the circuit composed of the ADC 1 and the compensation circuit 2 implements a conversion and compensation of the analog signal x(t) to form a sequence of compensated digital data yn.
A configuration phase is initiated at regular intervals to check the set of coefficients used, c1, . . . cK in terms of their current validity. In this way drifting nonlinear distortions caused for example by the circuit heating up or other interfering effects from the environment can be compensated.
The configuration phase is turned on and off as needed so that any slow changes in nonlinear distortions can be detected early enough and then compensated. It is, of course, in principle also possible to implement a predetermination and consideration of anticipated additional degradations or improvements related to the generation of distortions.
A control device 104 is advantageously employed to control the compensation circuit, the control device 104 being connected to a time-monitoring device, specifically, a timer 106. In addition to turning the configuration phase on and off, the control device 104 also controls the individual components through, for example, a bus 7.
In particular, it is possible to provide different types of test signals s(t), Sn for different application areas of the circuit so that coefficients c1, . . . cK may, for example, be optimally adjusted for a low-frequency or high-frequency analog signal x(t).
The following discussion examines in more detail the circuit components and the operational sequences of the method with reference to the mathematical background.
The configuration phase starts with an analog test signal s(t),
to which the theoretical uncorrupted sampling values




  
    
      S
      n
    
    =
    
      s
      ⁡
      
        (
        
          n
          Fs
        
        )
      
    
  
  ,




correspond after analog-to-digital conversion, without nonlinear distortion, which values are in turn to be output after optimal compensation from the compensation circuit 2 as the sequence of compensated digital data yn. Here n is a consecutive index of the set of natural numbers, while Fs is the sampling frequency of the ADC 1.

After analog-to-digital conversion has been performed on the analog test signal s(t) the sequence of digital data xn is supplied to its output based according to the expression

xn=sn+dn.

The sequence of digital data xn thus corresponds to the summation of correct theoretical and uncorrupted sampling values sn, and the respective distortion data value dn which matches the corresponding distortion by the A/D converter 1.

The digital compensation circuit 2 which uses the coefficients c1, . . . CK to generate the sequence of compensated digital signals yn that are ultimately to be output must therefore perform a compensation having the characteristic that may be described by a Kth-order polynomial:




  
    
      
        
          y
          n
        
        =
        
          
            
              ∑
              
                k
                =
                l
              
              K
            
            ⁢
            
              
                c
                k
              
              ·
              
                x
                n
                k
              
            
          
          =
          
            
              
                v
                ·
                
                  s
                  n
                
              
              +
              
                D
                n
              
            
            =
            
              
                S
                n
              
              +
              
                
                  D
                  n
                
                .
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




Here the coefficient values ck with k=1, 2, . . . K are adaptive coefficients, that is, coefficients which may be adjusted as necessary. The output signal, or sequence of outputted compensated digital data yn contains a map of the test signal or of the sequence of the digital test signal data Sn plus a possible change ν, specifically, amplification or attenuation. The sequence of the compensated digital data Sn is thus the product of a distortion factor ν and the sequence of the digital test signal data Sn which may be described by

Sn=v·sn,

where the effective distortions of the switching sequence of the ADC 1 and, in the event of insufficient compensation, of the compensation circuit 2 may be described by the sequence of difference data Dn according to the expression

Dn=yn−Sn.  (2)

Since the parameters sn of the test signal are known, the sequence of output signal data Sn may be extracted from the sequence of the compensated digital data yn by the test signal check device 4 to enable the actual distortion data or difference data Dn in data yn to be calculated at the output of the compensation circuit 2. In addition, the gradients of the rms distortion may be calculated using the expression




  
    
      ∂
      
        D
        n
        2
      
    
    
      ∂
      
        c
        k
      
    
  
  =
  
    
      2
      ·
      
        (
        
          
            y
            n
          
          -
          
            S
            n
          
        
        )
      
      ·
      
        x
        n
        k
      
    
    =
    
      2
      ·
      
        D
        n
      
      ·
      
        
          x
          n
          k
        
        .
      
    
  




The use of an iterative method allows the set of coefficients ck to converge. To this end, the formulation

ckn+1=ckn−G·Dn·xnk  (3)

may be selected so as ultimately to minimize the rms distortion or output. A parameter G is introduced in equation (3) as a stability criterion, which parameter at the same time provides for the highest possible convergence rate. The term ckn describes the value of the coefficients ck in the nth iteration step, the coefficient of compensation ck again having the consecutive index k=1, . . . , K. The iteration steps are preferably counted from the value zero, so that n=0, 1, . . . .

In an especially preferred embodiment, a sinusoidal test signal s(t)=sin[2πt] is preferably used as the test signal to perform the nonlinear compensation since it is then simpler for the A/D converter 1 to determine the structure of the distortions, and is possibly simpler for the compensation circuit 2 to determine additional distortions. A nonlinearly distorting ADC 1 having a sinusoidal input signal at a frequency Ft produces harmonics on frequencies p·Ft, which may be folded back by sampling to form




  
    f
    k
  
  =
  
    {
    
      
        
          
            
              
                (
                
                  p
                  ·
                  
                    F
                    t
                  
                
                )
              
              
                mod
                ⁢
                
                  
                
                ⁢
                Fs
              
            
          
          
            
              
                
                  (
                  
                    p
                    ·
                    
                      F
                      t
                    
                  
                  )
                
                
                  
                    mod
                    ⁢
                    
                      
                    
                    ⁢
                    Fs
                  
                  ⁢
                  
                    
                  
                  ≤
                
              
              ⁢
              
                
              
              ⁢
              
                
                  F
                  s
                
                2
              
            
          
        
        
          
            
              
                F
                s
              
              -
              
                
                  (
                  
                    p
                    ·
                    
                      F
                      t
                    
                  
                  )
                
                
                  mod
                  ⁢
                  
                    
                  
                  ⁢
                  Fs
                
              
            
          
          
            
              
                
                  (
                  
                    p
                    ·
                    
                      F
                      t
                    
                  
                  )
                
                
                  
                    mod
                    ⁢
                    
                      
                    
                    ⁢
                    Fs
                  
                  ⁢
                  
                    
                  
                  ≥
                
              
              ⁢
              
                
              
              ⁢
              
                
                  F
                  s
                
                2
              
            
          
        
      
      .
    
  




Here p=2, 3, . . . , M is the consecutive index of the frequency calculation.

If the first harmonics are significant, specifically, if p=2, . . . , M applies, the test frequency Ft should be selected such that the frequency band 2B is maximized around the fundamental where none of the first M harmonics fold back according to the expression




  
    
      
        B
        =
        
          
            max
            
              F
              t
            
          
          ⁢
          
            
              {
              
                
                  min
                  
                    
                      p
                      =
                      2
                    
                    ,
                    ...
                    ⁢
                    
                      
                    
                    ,
                    M
                  
                
                ⁢
                
                  
                
                ⁢
                
                  {
                  
                    
                    
                      
                        F
                        t
                      
                      -
                      fp
                    
                    
                  
                  }
                
              
              }
            
            .
          
        
      
    
    
      
        (
        4
        )
      
    
  




The extraction of the test signal or sequence of the output signal data Sn may be implemented using a known method of carrier processing. Another factor which must be taken into account is that the test signal s(t) or the sequence of the output signal data Sn assigned to this signal are amplitude-conforming.
To this end, a circuit may be used that is based on an I/Q demodulator (I: in-phase, Q: quadrature phase) and a Cordic circuit 43, and calculates the phase and amplitude of the input sequence of the digital data yn. FIG. 2 provides an example of this circuit.
Frequency, amplitude and direct current (DC) are recovered by feedback using the method known from automatic control engineering. A proportional and integral (PI) control is employed to determine the frequency. To determine amplitude and direct current, a PI-control is used in which only the proportional components are employed. The control parameters used are the P-component of the amplitude control Ca, the P-component for the DC components Cdc, and the P-component and I-component of the frequency control Cp or Ci, which components are intended at the same time to meet the stability criterion for a control loop and to ensure the fastest possible transient.
The supplied sequence of the digital data yn is input to multipliers 41a, 41b. A sinusoidal signal sin[2πt] as the signal sequence to be multiplied is fed to the first of the multipliers 41a by a sinusoidal tone generator 41. Analogously, a cosine signal sequence cos[2πt] is supplied by the sinusoidal tone generator to the second multiplier 41b. After multiplication, the two data sequences are each fed to an associated filter 42a, 42b, respectively, with undersampling P. After filtering, the I-separated and Q-separated signal components are supplied to a Cordic circuit 43 which determines, a corresponding amplitude and corresponding phase that are supplied through two outputs.
The sequence representing the amplitude is input to a subtractor 44a, which also receives a feedback signal on a line 202. The subtractor 44a provides a difference signal to multiplier 44b, which multiplies the difference signal by a coefficient value Ca, and the resultant product is output to a summer 44c. The summer 44c sums the product with the past value. The summer 44c provides a signal sequence to a delay element 44d, which provides an output signal to a multiplier 44e. The multiplier 44e also receives a cosine signal cos[2πt] and provides the resultant product on a line 204. This system is ultimately used to determine the sequence of the output signal data Sn that is then supplied to the difference-forming subtracter 6, which in turn has the sequence of the digital data yn applied to it through the second input.
The Cordic circuit 43 also outputs a corresponding phase or sequence of phase data on a line 206. This data is also fed to a circuit composed of a plurality of components 45. In the embodiment shown, this circuit is composed of two parallel multipliers 45b, 45a to which additionally a coefficient value Cp or a coefficient value Ci is supplied. The output signal of the multiplier 45a is fed to an adder 45c, the output signal of which is supplied both to an inverter (z−1) 45d and to a second adder 45e. The output signal of the inverter 45d is fed to the second input of the adder 45c. Through another input, the second adder 45e receives data from the multiplier 45b in which coefficient value Cp is up-multiplied for the phase data. The output data from this adder 45e are fed on line 208 to another adder 45f which has two additional inputs. A frequency ratio Ft/Fs including the test frequency Ft of the ADC 1 and the sampling frequency is supplied through the first additional input. An output value of the delay (z−1) 45g connected after the adder 45f is fed through the second input. Output values of this delay 45g are supplied as the timing variable t to an input of the sinusoidal tone generator 41.
In addition, a DC component DCn is filtered out and extracted from the sequence of digital compensated data yn outputted from the compensation circuit 2. To this end, the digital compensated data yn are supplied to a circuit 46 composed of a subtracter 46a, a multiplier 46b, a adder 46c, and delay (z−1) 46d. A control parameter Cdc is applied through the second input to the multiplier 46b. The control parameter Cdc determines the rate of the transient. The output data sequence from the delay 46d is supplied to the inputs of the subtracter 46a and the adder 46c, as well as to the subtracter 6 generating the difference data Dn.
Referring to FIG. 2, the frequency Ft is thus derived using a phase-locked loop (PLL). The input signal corresponding to the sequence of compensated digital data yn is split into I-component and Q-component that are then filtered by the low-pass filters 42 a and b of critical frequency B and undersampled. From the filtered I-component and Q-component, the Cordic circuit 43 then computes the amplitude and phase between input signal yn and the locally generated sinusoidal tone from the sinusoidal tone generator 41. The phase is passed to the PI control as the error signal. After a settling time, the sinusoidal tone generator 41 generates in its cosine branch a signal synchronized with the test tone. The coefficients Cp and Ci, and test frequency Ft as the known parameters determine the PI control. The amplitude is derived iteratively from the amplitude output of the Cordic circuit 43 using the control parameter Ca. The DC components are filtered and extracted from the output signal using the control parameter Cdc. The complete circuit composed of the test signal check device 4 (FIG. 1) and following the subtracter 6 (FIG. 1) finally generates a sequence of difference data Dn according to equation (3) proportional to the nonlinear distortions.
Advantageously, available carrier-processing systems and carrier-processing methods may be utilized for the purpose of implementing this circuit. What must be added are the circuits for the amplitude and DC components.
An especially preferred embodiment of the compensation circuit 2 is constructed segment-by-segment, as shown in FIG. 3.
The data sequence xn outputted by the ADC 1 is input to the compensation circuit 2 to a parallel system of multipliers 21. At the same time, the same input signal, i.e., once again the corresponding data value of the data sequence xn, is fed to the second input of first multiplier 212 so that a squaring is effected. The output of this first multiplier 212 is supplied to the input of the second multiplier 213, and so on, such that at each subsequent stage the exponent is increased by the value one up to a value xnK.
As a result, an exponentiation is effected, where each exponentiation step has an output so that values for digital data with exponentiations xn1, xn2, . . . , xnK are output from the input and the field of the multipliers 21. These are then fed to another field of multipliers 22, whereby a multiplication is performed with one each of the corresponding coefficients ck(m) where k=1, 2, . . . , K. What is described is thus a multi-element system with the coefficients ck(m) of the compensation in the mth segment with m=0, 1, 2, . . . , N−1 as the mth segment of the amplitude range. Accordingly, the nonexponentiated value or nonexponentiated data sequence xn1 as well as the coefficient c1(m) are entered in the first multiplier 221 of the second multiplication field 22. The once exponentiated data value xn2 and the second coefficient c2(m) are entered in the second multiplier 222, etc. The output values of the multipliers 221, 222, . . . , 22K of second multiplication field 22 are fed to an adder 23 which performs an addition of all input values, and also of the zeroth coefficient c0(m), then outputs the sequence of compensated digital data yn.
The sequence of digital data xn output by the ADC 1 is also fed to the coefficient determination system 5, where a rounding operation is performed in an index determination device 51, taking into account the N segments during the determination of the index m. Here the sequence of the digital data xn with its respective value increased by one is divided by two, then multiplied by the number of segments N. The thus generated segment index m on a line 302 is fed to a coefficient memory system 52 that is composed of a plurality of m parallel memory components 520, 521, . . . , 52N−1. The respective coefficients c1(m), c2(m), . . . , cK(m) und c0(m) are stored in the individual segments of this coefficient memory system 52. For each stored index m, there is an output to an adder 531, 532, . . . , 53K, 530, the output of which in turn is fed back to the same segment of coefficient memory system 52. The result from a multiplier 541, 542, . . . , 54K is entered in the second input of the adders 531, . . . . Each correspondingly exponentiated value of the sequence of digital data xn1, xn2, . . . , xnK is fed to the inputs of the multipliers 541, 542, . . . , 54K. Supplied to each second input of the multipliers 541, 542, . . . , 54K is the result of a multiplier 55 to which both the corresponding values for difference data Dn and the negative parameter −G serving as the stability criterion are supplied. Only the value for the difference data Dn, multiplied by the negative parameter −G, is supplied to the adder 530 at the adder's second input.
Using a circuit of this type, the compensation circuit 2 may be implemented on a segment-by-segment basis. The range of input data values, that is, the sequence of digital data xn, from the ADC 1 with data values from −1 to +1 is, uniformly distributed among N segments according to the expression




  
    
      
        -
        1
      
      +
      
        
          2
          N
        
        ⁢
        m
      
    
    ≤
    
      x
      n
    
    ≤
    
      
        -
        1
      
      +
      
        
          2
          N
        
        ⁢
        
          (
          
            m
            +
            1
          
          )
        
      
    
  
  ,




where the segment index m lies between 0 and N−1. As a result, one coefficient set {c0(m), c1(m), . . . , cK(m)} is assigned to each segment. Based on segmental interpolation of the characteristic, zeroth coefficients c0(m) are added in, so that for the sequence of compensated digital data yn outputted from the compensation circuit 2 the following expression applies:





  
    
      
        
          
            y
            n
          
          =
          
            
              
                ∑
                
                  k
                  =
                  0
                
                k
              
              ⁢
              
                
                  
                    
                      c
                      k
                    
                    ⁡
                    
                      (
                      m
                      )
                    
                  
                  ·
                  
                    x
                    n
                    k
                  
                
                ⁢
                
                  
                
                ⁢
                where
                ⁢
                
                  
                
                ⁢
                m
              
            
            =
            
              ⌊
              
                N
                ·
                
                  
                    
                      x
                      n
                    
                    +
                    1
                  
                  2
                
              
              ⌋
            
          
        
        ,
      
    
    
      
        (
        5
        )
      
    
  




where └┘ represents the rounding operation.

The equation (5) for compensation in the compensation circuit 2, and the iterative calculation of coefficients according to equation (3) may be effectively implemented together, as FIG. 3 illustrates. In a memory of size N×(K+1), here the coefficient memory system 52, N sets of K+1 coefficients each c1(m), c2(m), . . . , cK(m), and c0(m) are stored for the respective m=0, 1, 2, . . . , N−1. For each sampling instant, the index m is derived according to equation (5) from the input signal, i.e., from the applied value for the sequence of digital data xn, and assigned to the corresponding coefficient set 520, 521, . . . , 52N−1, then applied accordingly within the compensation circuit 2. Also in this procedure, the stored value for each coefficient is iteratively improved according to equation (3) and stored in the same memory location.
As shown in FIG. 4, in a certain case the method with segmental interpolation may be simplified with N=2B. For this purpose, it is necessary that the number N of segments agree with the resolution of the signal or the sequence of digital data xn, i.e., N=2B applies, where B is the number of bits per sampling value given a signal range between −2B−1 and 2B−1−1 and only one coefficient per segment, i.e., K=0.
The circuit of FIG. 3 is thereby reduced to the circuit of FIG. 4 which has, following the index determination device 51 for determining the index m, a look-up table with 2B adaptive coefficients c0(m) in memory fields 520, . . . , 52N−1. In this arrangement, all the multipliers 54 are eliminated so that only the first addition stage with adders 531, . . . remains, as described above.
Initial synthetic calculations using a sinusoidal tone as the test signal s(t) and a mathematical model of the analog-to-digital characteristic with frequencies Fs=40.5 MHz, Ft=1.84 MHz, the A/D model with coefficients for the calculation based on 0.9895x+0.0028x2+0.024x3−0.0064x4 and for compensation N=1 segments, and a maximum running index of m with K=4, produced significant improvements. After analog-to-digital conversion, the initial value was −60.3 dB, and after compensation the value was 80.9 dB—yielding a significant improvement of 20.5 dB. In the case of one measurement (K3), the improvement found was −47.2 dB after A/D conversion, as compared with −56.3 dB after compensation, giving an improvement of 9.1 dB. The ratio of distortions to outputs after A/D conversion was −46.8 dB, as compared with −55.3 dB after compensation—producing an improvement of 8.4 dB.
In place of a plurality of individual components, as described above, an implementation is also possible in an analogously capable computer chip, or in a monolithically fabricated semiconductor device in the form of an integrated circuit or the like.
Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.

## Claims

1. A system for compensating for distortion within an analog-to-digital converter, the system comprising:
a test signal generator that provides an analog test signal and a digitized test signal indicative of the analog test signal;
an analog-to-digital converter that selectively receives an input signal and the analog test signal and provides a digitized signal;
a compensation circuit that receives the digitized signal and filter coefficients, and provides a compensated digitized signal;
a test signal check device that receives and processes the compensated digitized signal to extract a sequence of output data;
a difference unit that receives the sequence of output data and the compensated digitized signal and provides a difference signal indicative of the difference; and
a coefficient determination unit that receives the difference signal and the digitized signal and provides the filter coefficients.

2. The system of claim 1, where the compensation circuit comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        ∑
        k
      
      
        k
        =
        0
      
    
    ⁢
    
      
        
          c
          k
        
        ⁡
        
          (
          m
          )
        
      
      ·
      
        
          x
          n
          k
        
        .

3. The system of claim 2, where the coefficients ck are computed as a function of the digitized signal.

4. The system of claim 1, where the compensation circuit comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        
          ∑
          k
        
        
          k
          =
          0
        
      
      ⁢
      
        
          
            
              c
              k
            
            ⁡
            
              (
              m
              )
            
          
          ·
          
            x
            n
            k
          
        
        ⁢
        
          
        
        ⁢
        
          where
        
        ⁢
        
          
        
        ⁢
        m
      
    
    =
    
      
        ⌊
        
          N
          ·
          
            
              
                x
                n
              
              +
              1
            
            2
          
        
        ⌋
      
      .

5. The system of claim 1, where the test signal check device comprises a CORDIC unit responsive to in-phase and quadrature components of the digitized signal yn to provide an amplitude signal to a DC component recovery circuit that provides the sequence of output data.

6. The system of claim 1, further comprising a timing unit that selectively determines whether the input signal or the analog test signal is processed by the analog-to-digital converter.

7. The system of claim 1, further comprising a timing unit that selectively determines whether the input signal or the analog test signal is processed by the analog-to-digital converter.

8. A system for compensating for distortion within an analog-to-digital converter, the system comprising:
means for providing an analog test signal;
an analog-to-digital converter that selectively receives an input signal and the analog test signal and provides a digitized signal;
compensation means for receiving the digitized signal and adaptive filter coefficients, and for providing a compensated digitized signal;
means for receiving and processing the compensated digitized signal to extract a sequence of output data;
a difference unit that receives the sequence of output data and the compensated digitized signal and provides a difference signal indicative of the difference; and
means responsive to the difference signal and the digitized signal for providing the filter coefficients.

9. The system of claim 8, where the compensation circuit comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        ∑
        k
      
      
        k
        =
        0
      
    
    ⁢
    
      
        
          c
          k
        
        ⁡
        
          (
          m
          )
        
      
      ·
      
        
          x
          n
          k
        
        .

10. The system of claim 9, where the coefficients ck are computed as a function of the digitized signal.

11. The system of claim 8, where the compensation circuit comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        
          ∑
          k
        
        
          k
          =
          0
        
      
      ⁢
      
        
          
            
              c
              k
            
            ⁡
            
              (
              m
              )
            
          
          ·
          
            x
            n
            k
          
        
        ⁢
        
          
        
        ⁢
        
          where
        
        ⁢
        
          
        
        ⁢
        m
      
    
    =
    
      
        ⌊
        
          N
          ·
          
            
              
                x
                n
              
              +
              1
            
            2
          
        
        ⌋
      
      .

12. The system of claim 8, where the means for receiving and processing comprises a CORDIC unit responsive to in-phase and quadrature components of the digitized signal yn to provide an amplitude signal to a DC component recovery circuit that provides the sequence of output data.

13. The system of claim 8, further comprising a timing unit that selectively determines whether the input signal or the analog test signal is processed by the analog-to-digital converter.

14. A system for compensating for distortion within an analog-to-digital converter, the system comprising:
a test signal generator that provides an analog test signal;
an analog-to-digital converter that selectively receives an input signal and the analog test signal and provides a digitized signal;
compensation means for receiving the digitized signal and filter coefficients, and for providing a compensated digitized signal;
means for receiving and processing the compensated digitized signal to extract a sequence of output data;
a difference unit that receives the sequence of output data and the compensated digitized signal and provides a difference signal indicative of the difference; and
means responsive to the difference signal and the digitized signal for providing the filter coefficients.

15. The system of claim 14, where the compensation means comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        ∑
        k
      
      
        k
        =
        0
      
    
    ⁢
    
      
        
          c
          k
        
        ⁡
        
          (
          m
          )
        
      
      ·
      
        
          x
          n
          k
        
        .

16. The system of claim 15, where the coefficients ck are computed as a function of the digitized signal.

17. The system of claim 14, where the compensation means comprises a compensator unit that provides the compensated digitized signal yn based upon the expression




  
    y
    n
  
  =
  
    
      
        
          ∑
          k
        
        
          k
          =
          0
        
      
      ⁢
      
        
          
            
              c
              k
            
            ⁡
            
              (
              m
              )
            
          
          ·
          
            x
            n
            k
          
        
        ⁢
        
          
        
        ⁢
        
          where
        
        ⁢
        
          
        
        ⁢
        m
      
    
    =
    
      
        ⌊
        
          N
          ·
          
            
              
                x
                n
              
              +
              1
            
            2
          
        
        ⌋
      
      .

18. The system of claim 14, where the menas for receiving and processing comprises a CORDIC unit responsive to in-phase and quadrature components of the digitized signal yn to provide an amplitude signal to a DC component recovery circuit that provides the sequence of output data.